It depends what is in the Excel file, but you could presumably create a new XLS with just the email address, save it as a .csv file and then import it into your address book, then email out in suitable sized chunks (e.g. alphabetically).
Do you want to personalise each email?
In Word 2002 and above (possibly earlier also) you can do a merge to email. The best source is the Help within Word.
Getting the data from XL to Word is easy, depending on whether you want it as a table or a delimited text file - e.g. CSV.
We've had threads here before on mail merges - search for mail merge in this forum and any date.
SD